Hi Igor,

Check your physical path for a directory named "subportal" (i.e.; c:\inetpub\wwwroot\myserver\subportal).  This directory (and the default.aspx file inside of it) is necessary for DNN to receive an opportunity to handle the incoming request. 

If it's not there, then you're probably missing some combination of directory create permissions on your virtual directory.  It's safe to just go ahead and copy this file up from your dev machine onto production.

If I recall correctly, the directory is created at the time of subportal creation. 

Hope this helps!

Brandon